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ASTM F2129 - Electrochemical Corrosion Testing of Surgical Implants

 

BACKGROUND 

Six (6) metallic specimens were submitted for electrochemical evaluation in accordance with ASTM F2129 (Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices.)  The surgical implant materials received were identified as follows: 

 

TEst procedure

In the electrochemical technique chosen, cyclic potentiodynamic polarization, a metal specimen (working electrode) is exposed to the solution in a test cell fitted with a reference electrode (saturated calomel) and a counter electrode (graphite).  The potentiostat is connected to these three electrodes, and the potential of the working electrode, with respect to the reference, is scanned through a voltage range from negative to positive, and then reversed to its starting potential.  The resulting current between the working and counter electrodes is recorded.  This scan is then plotted on a semi-log graph, allowing the resulting curve to be analyzed for key-point voltages and currents. 

An electrical lead was connected to each specimen.  This lead allowed both electrical connection to the potentiostat as well as suspension in the test solution. During testing, partial masking of the specimen was required because of the need to isolate the metal to metal contact of the lead.  This eliminated the possibility of any galvanic interference. 

Since the specimens submitted are ultimately to be used in the human body, a phosphate buffered saline solution (pH 7.4) was used as an electrolyte for testing.  The specimens were tested in the as received condition.  Each specimen was inserted in the test chamber, which was filled with solution, and suspended such that the specimen remained below the surface of the solution.  The end fitted with the test lead hung just below the liquid/air interface.  The cell was allowed to equilibrate for approximately 1 hour prior to performing the polarization test.   

The conditions used for the F2129 electrochemical tests were as follows: 

Solution

Phosphate Buffered Saline (pH 7.4)

Temperature

37°C

Agitation

No

Gas Purge

150 cc/min. nitrogen

Scan Rate

0.167 mV per second

 

RESULTS

The key-point voltage and current data are summarized in Table 1, and the calculated corrosion rate and localized attack propensity are presented in Table 2. 

 

Table 1

Key-Point Electrochemical Data *

Polarization Scan

Eoc

Eb

Ep

Icorr

Eb/Ib

Hysteresis

1

5

---

---

0.01

---

No

2

-35

---

---

<0.01

---

No

3

-217

430

<-317

0.03

<0.01

Yes

4

-236

377

-300

0.04

<0.01

Yes

5

-207

329

<-308

0.05

<0.01

Yes

6

-225

354

-147

0.12

<0.01

Yes

* NOTE:  E-values are in millivolts vs. SCE; and I-values are in microamperes (10-6 amps)

 

Table 2

Corrosion Rates and Localized attack Propensity

Polarization Scan

Corrosion

Rate (mpy)

Pitting and Crevice

Propensity

1

<0.1

Not Likely

2

<0.1

Not Likely

3

<0.1

Probable

4

<0.1

Probable

5

<0.1

Probable

6

<0.1

Probable

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